I46 MODERN PAPER-MAKING
of which one set is kept clean and ready to replace the other. This will save a
few minutes' time when washing out. The 'hairy' surface of the felt is very
effective in holding back sand and all solid particles. Sometimes 'ladders'
are laid in the bottoms of shutes. They are not much good and generally
become sources of dirt through neglect. Some mills, making fine papers,
have installed electro-magnets in their sand traps.
These magnets attract and retain all particles of iron, and even rust, so
effectively that the rusty specks and spots so often seen in tub-sized papers are
entirely eliminated. Strangely enough, specks of all kinds of metal seem to
be attracted and stick about the poles of the magnet. A good substitute for
this machine can be made by magnetising a number of old files and arranging
them on the bottom of the shutes. Though not by any means so powerful
as the- electro-magnet, these magnestised files help to do their bit in giving
clean paper.
The sand traps may be utilised to catch a little of the paper-maker's worst
enemy—rubber. BA board or boards fixed with their under edges just below
the level of the stuff, across the shutes, retain a great deal of scum or froth in
which may be found specks of rubber that have floated to the surface. If these
.boards are inclined at an angle away from the flow of the stuff, floating particles
of all kinds will be washed up and stranded on their sloping surfaces.
It is generally agreed that the longer the shutes the more chance has the
stuff to get rid of dirt, but there is a limit which must be set for economical
reasons. Where very frequent changes of sorts are necessary, long shutes may
be wasteful both in stuff and time, since every alteration of stuff or water alters
the level of the stuff flow, and may even cause the dirt to be disturbed and
begin to come through with the stuff. When this happens, the only cure is
to shut and wash out the sand traps. The best plan for die machine is to have
about 50 yards of shutes, with a by-pass at 25 yards, and a short cut for very
thick or wet beaten papers. Banks and all thin papers may be run with the
full length, as they have proportionately a heavy flow of water with the stuff
Medium weights and small orders -may be run the 25 yards, and those papers
which must be run with very little water take the shortest way. If it is
attempted to run the latter class through too long shutes, the flow will be
sluggish, and most probably will follow a narrow channel in the centre, leaving
thick stuff stagnant at the sides. In connection with this, it may be mentioned
that when the machineman opens the water gate to run more water with the
stuff, the weight of the sheet increases for a few minutes, owing to the slight
flooding effect driving the stuff in front of it. Conversely, if he shuts off
water, the weight falls temporarily, owing to the checking of the flow. It
is, therefore, important that the machineman should keep accurate note of the